MECHANISM OF PHOTOSYNTHESIS

 The process of Photosynthesis is a complicated oxidation-reduction process ultimately resulting in the oxidation of water and reduction of CO2 to carbohydrates.

The mechanism of photosynthesis can be studied with the following two processes :

I. Primary Photochemical Reaction or Light reaction or Hill’s reaction, and

II. Dark reaction or Blackman’s reaction or Path of carbon in photosynthesis

Importance of two processes photosynthesis

In the primary photochemical reaction, assimilatory powers (NADPH2 + ATP) are generated and O2 is released. These assimilatory powers are utilized in the dark reaction during which process CO2 is reduced to carbohydrates

I. PRIMARY PHOTOCHEMICAL REACTION (OR) LIGHT REACTION (OR) HILL’S REACTION (Activities found in thylakoids or grana)

In 1937, Robert Hill demonstrated that isolated chloroplasts evolved O2 when they were illuminated in the presence of suitable electron acceptors, such as ferricyanide. The ferricyanide is reduced to ferrocyanide by photolysis of water. This reaction is now called as Hill Reaction.

Photolysis of water or Hill Reaction (oxidation of water)

Splitting a water molecule into OH- and H+ ions in the presence of light is called as Photolysis of water. The OH- ions unite to form water molecules again and release O2 and electrons (e-). While, the H+ is used in the formation of energy-rich molecules, NADPH2 during light reaction. It explains that water is used as a source of electrons for CO2 fixation and O2 is evolved as a by-product.

It is believed that photolysis of water involves a strong oxidant, designated as “Z”.

The primary photochemical reaction is faster than the dark reaction. It takes place only in the presence of light in the grana portions of the chloroplasts. It is a complex process with several important events.

Primary Photochemical Reactions can be studied under the following steps :

A. Red drop, Emerson Enhancement Effect, and Two Pigment Systems :

i. Red Drop and Emerson Enhancement Effect (EEE)

ii. Two Pigment Systems.

B. Production of Assimilatory Powers

i. Electron Transport System in Photosynthesis or Reduction of NADP

ii. Photophosphorylation

They are three kinds of Photophosphorylation, viz.,

i. Cyclic Photophosphorylation

ii. Non-cyclic Photosphorylatioh, and

iii Pseudocyclic Photophosphorylation

C. Energy relationships and efficiency of photosynthesis

D. Interrelationship between light and dark reactions

A. Red drop, Emerson’s Enhancement Effect (EEE), and Two Pigment Systems

Emerson and Lewis (1943) observed a sharp decrease in the quantum yield of photosynthesis at wavelengths longer than 680nm. Because this decrease takes place in the red spectrum of the light, they called this drop a Red Drop.

However, Emerson and Chalmers (1951) found that this inefficient wavelength of 680 nm could be made to be fully efficient if supplemented with the light of shorter wavelengths. They also observed that the quantum yield from the combined beams of light (red and far-red of shorter and longer wavelengths respectively) was found to be greater than the sum effect of both beams when used separately. This enhancement of the rate of photosynthesis is called Emerson’s Enhancement Effect.

ii. Two Pigment Systems

With the discovery of Red Drop and Emerson Enhancement Effect, it is concluded that at least two pigment systems are involved in photosynthesis. These two pigment systems are known as Photosystem I (PS I) and Photosystem II (PS II). The wavelengths of light longer than 680 nm influence only PS I. While, wavelengths of light shorter than 680nm influence both the PS I and PS II.

Components of Photosystem I :

PS I complex consists of :

* ∼ 200 chlorophylls

* ∼ 50 carotenoids

* one molecule of P 700

* one cytochrome f(cyt.f)

* cyt. b 563

* FRS (Ferredoxin Reducing Substance)

* One or two membrane-bound ferredoxin molecules etc.

* It is also rich in chl a iron and copper

* PS I controls the process of producing a strong reductant to reduce NADP and NADPH2, besides

producing ATP molecules.

Components of Photosystem II :

* ∼ 200 chlorophylls

* ∼ 50 carotenoids

* a molecule of P 680

* a primary electron acceptor Q

* a Plastoquinone

* four plastoquinone equivalents

* four Mn molecules bound to one or more proteins

* two cyt.b 559

* one cyt.b6 and

* Chloride

* PS II is concerned with the generation of strong oxidants and weak reductants coupled with the release of O2.

B. Production of Assimilatory Powers

Arnon (1956) used the term assimilatory powers to refer to ATP (Adenosine TRi-Phosphate) and NADPH2 (reduced Nicotinamide Adenine Dinucleotide Phosphate) molecules.

The process of reduction of NADP into NADPH2 may be denoted as Electron Transport System (ETS) in photosynthesis or Reduction of NADP. The process of formation ATP from ADP and inorganic phosphate (Pi) in light reaction of photosynthesis is called Photosynthetic Phosphorylation or Photophorylation.

i. Electron Transport System (ETS) or Reduction of NADP

The ETS involves the transport of electrons from water and NADP to form NADPH2. The H+ released during the

process of photolysis of water reduces NADP to NADPH2 during the process of non-cyclic photophosphorylation.

When chl a molecule receives a photon of light, it becomes excited and expels and extra energy along with an electron in both the pigment systems (PS I & II). This electron, after traveling through a number of electron carriers, is either cycled back or consumed in reducing NADP (Nicotinamide Adenine Dinucleotide Phosphate) to NADPH2. The extra light energy carried by the electron is utilized in the formation of ATP molecules at certain places during its transport.

ii. Photophosphorylation

Arnon has contributed a lot to the understanding of electron transport and phosphorylation in chloroplasts.

They are of two types

1. Cyclic photophosphorylation

2. Non-cyclic photophosphorylation.

1. Cyclic Photophosphorylation

This is a pathway of ATP formation during the light reaction of photosynthesis. It involves only PS I and the wavelength of light greater than 680 nm.

* In cyclic photophosphorylation, activation of PS I by wavelength greater than 680 nm (P700) causes the electron to flow from P700 to ferredoxin to cyt-b6 which in turn passes back the electrons to P700 via cyt-f and plastocyanin.

* The synthesis of ATP in this electron transport is possible at two places, i.e., between ferredoxin and cyt-b6 and cyt-f, by means of phosphorylation of one ADP molecule to form one ATP molecule in each place.

* During cyclic photophosphorylation, 2ATP molecules are produced per electron transfer. Since the electron which was ejected from P700 in the above electron transport system is cycled back, the process is called Cyclic Electron Transport and the accompanying phosphorylation as the Cyclic Photophosphorylation.